BAGHDAD, Iraq — Gunmen ambushed a bus carrying dozens of cleaners and other workers from a Shiite Muslim district in Baghdad to the city’s airport, killing at least 15 people and wounding 15 others, a hospital source said.

The source said 15 bodies and 15 wounded people had been brought to the hospital after the attack in the Sunni Arab neighborhood of Amriya in western Baghdad.

An Interior Ministry source confirmed there was an attack on a bus in the area and said initial reports were of four dead and nine wounded, but the toll could rise.

A boy aged around 15 told a Reuters reporter on the outskirts of the airport that he was one of five or six people to escape unhurt when the gunmen attacked the bus.

“All my colleagues were shot, we don’t know where the bullets came from, they came from everywhere,” said the boy, who gave his name only as Karar. He was crying and his clothes were covered in mud.

“I survived and some of my colleagues, around five or six people. All the others were lying on the ground, I don’t know if they were killed or wounded,” he said.

The Transport Ministry, which controls the airport, is run by supporters of Shiite cleric Moqtada al-Sadr, whose Mehdi Army militia is blamed by Washington and Sunni Arabs for operating death squads.

Hundreds of people are killed every week in sectarian attacks, and buses carrying workers have frequently been targeted by kidnappers and gunmen.

Security sources say the main road to the airport, which also houses the main U.S. military base in Baghdad, has seen a rise in insurgent activity in recent days.

It used to be among the most dangerous routes in Baghdad but a crackdown by U.S. and Iraqi forces over the past year had cut the number of attacks. The highway passes through some of the most dangerous areas in Sunni Arab western Baghdad.

Sectarian tension has been heightened by the hanging of Saddam Hussein at the end of last year and by an illicitly filmed video showing the Sunni Arab former president being taunted by followers of Sadr.